vented crawlspace/insulation in floor joist

Long post, so please bare with me while I try to explain everything.

I live in a doublewide mobile home in eastern NC. Bricked under with 10 manual perimeter vents. Dry underneath, no issues with water or moisture in crawlspace. I had to remove the heating ducts (long story), so underneath is totally open now.

Pictures show 1. what it looked like before, with the belly paper(actually like 6 ml plastic) holding in the insulation, waterlines, ductwork. 2. what it looks like now that insulation and heat ducts are gone.

Floor joists are 2 x 6. I want to reinstall insulation. It is not feasible to seal vents and install foamboard inside perimeter brick because of the way the concret supports? are underneath (very hard to manuver around).

I have read the building science website regarding crawlspace insulation.
It is also not feasible for me to install the sheathing to the underside of the floor joist (as in b.s. picture). I would have to cut the sheathing down to 2 ft sections to get them through the crawlspace entry box?

My questions are: 1. Should I use insulation made for 2 x 4 walls or 2 x 6 walls? 2. Should kraft paper face up into the house(warm area)?

I ask about the insulation for 2 x 4 walls because this is what the b.s. website shows. Subfloor, air space, insulation, sheathing.
I will staple the belly paper back to the underside of the floor joist and tape seams.

If I use the insulation for 2 x 6 walls, then the whole floor joist cavity will be filled, then the belly paper will be stapled to the underside of the joist.

Thank you for any help you can give me.

Figure 7: Cavity Insulation with Vapor Barrier—Adding impermeable foil faced insulating sheathing over fiberglass cavity insulation is a hybrid approach that uses the best qualities of both materials. Note that the optimum location for the airspace is above the cavity insulation. Are you folks paying attention at the EPA Energy Star Program? Makes for warmer floors—this is the same detail that should be used under bedrooms over garages.

Hold it up with wires or foam board. The faced f.g. goes toward the heated side in Winter. I would use poly only on the dirt as a vapor barrier.

Gary

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17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?

Gary,
Are you saying to use 2 x 4 insulation & hold it in place with wire?
If 2 x 4 insulation is used, which side should the air gap be on, the subfloor side or the ground side.

Why would it be improper to staple the belly paper to the underside of the floor joist to hold the insulation?

I forgot to add that I will be replumbing with pex, so there is no need to have the waterlines in the insulation.

I will also at a later date have a new house type HVAC installed, so there is no need to have the duct work in the insulation.

My objective is to replace everything I can with house materials. I have already installed an artic white metal roof, house windows with low-e & argon, house exterior & storm doors, house vinyl siding. Buying trailer stuff is way more expensive & most if not all is made in china. If something needs fixing, I want to be able to go to the local hardware store to buy a replacement part.